Method of improving fire tube burner efficiency by controlling combustion air flow and an air damper for a fire tube

ABSTRACT

A method of improving natural draft fire tube burner efficiency by controlling combustion air flow. The method involves positioning in a fire tube an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings. Air flow through the air damper body is controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate. The air damper body has an air inlet face, an air outlet face, and an outer circumference. The method involves establishing and maintaining a relationship between an air damper open area, firing rate and excess air to maintain stack oxygen below 3% in accordance with a formula A2+A3&lt;DR×FR×A1.

Method of improving fire tube burner efficiency by controlling combustion air flow and an air damper for a fire tube.

FIELD

There is described a method of improving natural draft fire tube burner efficiency by controlling combustion air flow and an air damper in accordance with the teachings of the method.

BACKGROUND

British Patent GB 1,603,618 contains a good summary of the theory behind combustion air control for burners. Fuel supply and air supply for a burner are controlled separately in stoichiometric proportions. It is necessary to have some excess air to ensure that combustion is complete, if the air and fuel are not perfectly mixed. However, it has been determined that burner efficiency drops if too much excess air is provided, as it takes energy to heat the excess air without producing any useful effect. For example, if the ambient air temperature is 40 degrees Fahrenheit and the burner tube stack temperature is 700 degrees Fahrenheit, excess air is heated by 660 degrees Fahrenheit. It is, therefore, best to have the smallest possible amount of excess air, while still ensuring that there is complete combustion. Air supply control is implemented by means of an air damper.

U.S. Pat. No. 4,702,692 (Burns et al) titled “Air Reduction Controls for Oil-Treating Vessels”, describes an air damper for a natural draft fire tube for use in the oil industry. This air damper consists of a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings. By rotating the rotatable plate, the air flow openings in the rotatable plate can either be brought into register with the air flow openings in the fixed plate or the air flow openings in the fixed plate can be at least partially blocked by the rotatable plate. The air damper of Burns et al was welded in a duct that extended radially from a fire tube.

U.S. Pat. No. 4,383,820 (Camacho) titled “Fuel Gas Burner and Method of Producing a Short Flame”, describes placing blades in an annulus of a burner to create a turbulent swirling action.

SUMMARY

According to one aspect there is provided a method of improving natural draft fire tube burner efficiency by controlling combustion air flow. The method involves positioning in a fire tube an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings. Air flow through the air damper body is controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate. The air damper body has an air inlet face, an air outlet face, and an outer circumference. The method involves establishing and maintaining a relationship between an air damper open area, firing rate and excess air to maintain stack oxygen below 3% in accordance with a formula A2+A3<DF×FR×A1, in which:

A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate;

A2=total area of other openings in the air damper body through which air can bypass the air flow openings;

A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and

FR=firing rate in % of maximum for a particular firetube-burner combination;

DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92.

According to another aspect, there is provided an air damper for a natural draft fire tube which includes an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings. Air flow through the air damper body is controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate. The air damper body having an air inlet face, an air outlet face, and an outer circumference; and

a relationship is maintained between an air damper open area, firing rate and excess air in accordance with a formula A2+A3<DF×FR×A1, in which:

A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate;

A2=total area of other openings in the air damper body through which air can bypass the air flow openings;

A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and

FR=firing rate in % of maximum rate for a particular firetube-burner combination;

DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92.

As will hereinafter be further described, the use of circumferential seal has a dramatic beneficial effect on performance, when compared to the same assembly without a circumferential seal. It is, therefore, preferred that there is a deformable circumferential seal around the outer circumference of the air damper body. Making the circumferential seal deformable accommodates “out of round” shape imperfections and surface imperfections, such as weld deposits, of the inner circumference of the fire tube. It is also preferred that the circumferential seal engage an inner circumference of a fire tube solely by friction. This facilitates ease of servicing and replacement.

A common industry approach was to leave a ¼ inch gap around the circumference of the air damper body. As will be apparent from the test data set forth below, it is now realized that this and other gaps rendered the air damper body ineffectual, as air would bypass the air damper body dictated solely by fire tube draft. As a result, at less than 100% maximum combustion rate, stack oxygen was always greater than 3%. Greater than 3% stack oxygen reduces fire tube burner efficiency and results in unnecessary carbon dioxide emissions.

There will hereinafter be described further beneficial features of the air damper. For example, an ignitor passage extends through the air damper body in parallel spaced relation to the central burner passage. This allows an ignitor to be inserted and removed. This is a useful feature as ignitors frequently need replacing.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:

FIG. 1 is a front elevation view of an air damper for a fire tube.

FIG. 2 is a front perspective view of the air damper illustrated in FIG. 1.

FIG. 3 is a rear elevation view of the air damper illustrated in FIG. 1.

FIG. 4 is a rear perspective view of the air damper illustrated in FIG. 1.

FIG. 5 is a top plan view of the air damper illustrated in FIG. 1.

FIG. 6 is a rear perspective view of the air damper illustrated in FIG. 1, positioned in a fire tube.

FIG. 7 is a front perspective view of the air damper illustrated in FIG. 1, positioned in a fire tube.

FIG. 8 is a side elevation view, in section, of the air damper illustrated in FIG. 1, positioned in a fire tube.

FIG. 9 is a detailed side elevation view, in section, of the air damper illustrated in FIG. 1 positioned in a fire tube.

DETAILED DESCRIPTION

An air damper, generally identified by reference numeral 10, will now be described with reference to FIG. 1 through FIG. 9.

Structure and Relationship of Parts:

Referring to FIG. 5, air damper 10 includes an air damper body 12 with a fixed plate 14 and a rotatable plate 16. Referring to FIG. 1 and FIG. 2 fixed plate 14 has a plurality of air flow openings 18. Referring to FIG. 3 and FIG. 4, rotatable plate 16 also has a plurality of air flow openings 20. Referring to FIG. 5, air damper body 12 has an air inlet face 22, an air outlet face 24, and an outer circumference 26. Air, indicated by arrows 28, enters air damper body 12 through air inlet face 22 and exits air damper body 12 through air outlet face 24. Referring to FIG. 2, FIG. 4 and FIG. 5, a central burner passage 30 is provided through air damper body 12. Referring to FIG. 7, central burner passage 30 is adapted to receive a burner body 100, as will hereinafter be further described. Referring to FIG. 1 through FIG. 4, a circumferential seal 32 is provided around outer circumference 26 of air damper body 12 to prevent natural draft air bypassing the air flow openings 18 and 20. Referring to FIG. 8 and FIG. 9, circumferential seal 32 is adapted to engage an inner circumference 102 of a fire tube 104 solely by friction. Referring to FIG. 1 through FIG. 4, a seal 31 is also provided to prevent natural draft air bypassing the air flow openings 18 and 20 through central burner passage 30.

Referring to FIG. 2, FIG. 4 and FIG. 5, it is preferred that rotatable plate 16 is positioned at inlet face 22 of air damper body 12 and that fixed plate 14 is positioned at outlet face 24 of air damper body 12. With this configuration, the rotatable plate is more readily accessed for the purpose of making adjustments.

Referring to FIG. 2 and FIG. 5, it is preferred that outlet face 24 of air damper body 12 has outwardly projecting air deflectors 34 overlying air flow openings 18 of fixed plate 14. This configuration imparts a helical flow to air flowing through air damper body 12.

Referring to FIG. 4 and FIG. 5, it is preferred that a handle 36 is provided on rotatable plate 16. This enables a manual force to be exerted via handle 36 to rotate rotatable plate 16 thereby adjusting the position of air flow openings 20 of rotatable plate 16 relative to air flow openings 18 of fixed plate 14.

Referring to FIG. 4 and FIG. 5, in order to prevent air flow from circumventing air damper body 12, fixed plate 14 and rotatable plate 16 are loosely clamped together. The term “loosely is used, as the mode of clamping must not impede rotation of rotatable plate 16. The mode of clamping illustrated are nuts 40 and bolts 42. Bolts 42 extend through fixed plate 14. However, to facilitate rotation of rotatable plate 16, bolts 42 extend through slots 44 in rotatable plate 16.

Referring to FIG. 2, FIG. 4 and FIG. 5, It is preferred that an ignitor passage 46 extend through air damper body 12 in parallel spaced relation to central burner passage 30. This configuration enables an ignitor 106 to be positioned to ignite combustion gas flowing through burner body 100. Referring to FIG. 1 through FIG. 4, a seal 47 is provided to prevent natural draft air bypassing the air flow openings 18 and 20 through ignitor passage 46.

Operation:

Referring to FIG. 6 through FIG. 8, burner body 100 has a nozzle end 108 and a fuel gas source attachment end 110. In preparation for installation, burner body 100 is inserted into central burner passage 30 of air damper body 12 with nozzle end 108 protruding past air outlet face 24 of air damper body 12 and fuel gas source attachment end 110 protruding past air inlet face 22 of air damper body 12. Air damper 12 is then inserted into fire tube 104. When air damper 12 is inserted into fire tube 104, circumferential seal 32 engages inner circumference 102 of fire tube 104 solely by friction. Referring to FIG. 9, it can be seen that circumferential seal 32 deforms to create an air seal. Beneficial results have been obtained when circumferential seal 32 is made of a flexible, high temperature rated material. Referring to FIG. 8, either before or after insertion of air damper body 12 into fire tube 104 ignitor 106 extended through ignitor passage 46 and positioned relative to nozzle end 108 to ignite combustion gas flowing through burner body 100 to nozzle end 108.

Tests were conducted to determine the relative efficiency of air damper 10, with and without circumferential seal 32. It was determined that air damper 10 with circumferential seal 32 outperformed air damper 10 without circumferential seal 32. The increase in total efficiency (BTUs into the process for BTUs created from gas combustion) depended upon the turn down rate of the combustion system. For example, when the combustion system firing rate was reduced to 40% of the maximum possible firing rate (60% turn down) and rotatable plate 14 was rotated relative to fixed plate 14 to reduce the air flow through air damper body 12 appropriately, air damper 10 with circumferential seal 32 transferred up to 43% more heat into the process for the same quantity of gas consumed, as compared to combustion assemblies having air dampers without circumferential seal 32.

The gap about the periphery of the damper had never previously been considered a problem because the relationship between air damper uncontrolled open area, controllable secondary air flow area, excess air and firing rate was not well understood. A peripheral seal was added, forcing all the air flow to pass through the damper. The effect on efficiency was then measured. Marginal increases in efficiency were measured at lower turn down rates. However, as the turn down rates became higher, unexpected increases in efficiency were measured. As set forth above, with a 60% turn down rate, up to 43% more heat is transferred. It was then realized that the other openings through the damper were also having an effect upon burner efficiency.

It is known that stack oxygen in a 2-3% range is sufficient to promote combustion without adversely affecting burner efficiency. It has been determined that a relationship exists between air damper open area, firing rate (turndown) and excess air. A secondary air control plate (damper) should be designed in accordance with a formula A2+A3<DF×FR×A1, in which:

A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate;

A2=total area of other openings in the air damper body through which air can bypass the air flow openings;

A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and

FR=firing rate in % of maximum rate for a particular firetube-burner combination;

DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92. (The symbol ̂ reflecting a raising to the power of 2)

This formula is applicable for all firing rates below 100% and natural draft greater than −0.05 mm h20. Stated another way, if A2+A3>DF×FR×A1, then there will no longer be excess air control and stack oxygen levels will be dictated by fire tube draft alone and will always exceed 3%. There follows a series of graphs and a summary of results showing a test comparison of five different models operating at 100% of Maximum Combustion Rate (MCR), 80% MCR, 60% MCR and 40% MCR. As will become apparent from a review of the test data set forth below, the differences in efficiency become more pronounced as the MCR is lowered. At 40% MCR, a burner controlled in accordance with the formula A2+A3<DF×FR×A1 uses 43% less fuel gas to provide the same heat input to the process vessel than other burners having dampers where A2+A3>DF×FR×A1. It is believed that the reason for this is that, notwithstanding the presence of a damper, too much excess air is drawn through various openings as dictated by fire tube draft. This formula is believed to be applicable to all natural draft tubes with a nominal outer diameter of 10 inches to 30 inches.

In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.

The scope of the claims should not be limited by the illustrated embodiments set forth as examples, but should be given the broadest interpretation consistent with a purposive construction of the claims in view of the description as a whole. 

What is claimed is:
 1. An air damper for a natural draft fire tube, comprising: an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings, with air flow through the air damper body being controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate, the air damper body having an air inlet face, an air outlet face, and an outer circumference; and a relationship is maintained between an air damper open area, firing rate and excess air in accordance with a formula A2+A3<DF×FR×A1, in which: A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate; A2=total area of other openings in the air damper body through which air can bypass the air flow openings; A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and FR=firing rate in % of maximum rate for a particular firetube-burner combination; DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92.
 2. The air damper of claim 1, wherein a central burner passage through the air damper body, the central burner passage being adapted to receive a burner body.
 3. The air damper of claim 1, wherein a deformable circumferential seal around the outer circumference of the air damper body, the circumferential seal being adapted to engage an inner circumference of a fire tube solely by friction.
 4. The air damper of claim 1, wherein the rotatable plate is positioned at the inlet face of the air damper body and the fixed plate is positioned at the outlet face of the air damper body.
 5. The air damper of claim 4, wherein the outlet face of the air damper body has outwardly projecting air deflectors overlying the air flow openings of the fixed plate.
 6. The air damper of claim 4, wherein a handle is provided on the rotatable plate, whereby a manual force is exerted via the handle to rotate the rotatable plate thereby adjusting the position of the air flow openings of the rotatable plate relative to the air flow openings of the fixed plate.
 7. The air damper of claim 2, wherein an ignitor passage extends through the air damper body in parallel spaced relation to the central burner passage, wherein an ignitor can be inserted and removed, a seal being provided to prevent airflow through the ignitor passage bypassing the air damper.
 8. The air damper of claim 2, in combination with a burner body, the burner body being positioned within the central burner passage of the air damper body, the burner body having a nozzle end protruding passed the air outlet face of the air damper body and a fuel gas source attachment end protruding passed the air inlet face of the air damper body.
 9. The air damper of claim 8, in combination with a fire tube.
 10. An air damper for a natural draft fire tube having an inner circumference and a burner body disposed therein, the air damper comprising: an air damper body comprising: a burner passage adapted to receive the burner body; a peripheral seal adapted to engage the inner circumference of the fire tube; and a plurality of air flow openings disposed about the central burner passage with the size of the air flow openings adjustable; wherein, in use, air flow between the central burner passage and the burner body is impeded, air flow between the peripheral seal and the inner circumference of the fire tube is impeded, and air flow through the air flow openings is user adjustable with a relationship maintained between an air damper open area, firing rate and excess air in accordance with a formula A2+A3<DF×FR×A1, in which: A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate; A2=total area of other openings in the air damper body through which air can bypass the air flow openings; A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and FR=firing rate in % of maximum rate for a particular firetube-burner combination; DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92.
 11. A method of improving natural draft fire tube burner efficiency by controlling combustion air flow, comprising: positioning in a fire tube an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings, with air flow through the air damper body being controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate, the air damper body having an air inlet face, an air outlet face, and an outer circumference; and establishing and maintaining a relationship between an air damper open area, firing rate and excess air to maintain excess air below 3% in accordance with a formula A2+A3<DF×FR×A1, in which: A1=total area of air flow openings in the air damper body created by a relative positioning of the air flow openings of the fixed plate and the air flow openings of the rotatable plate at a given closing position for a given firing rate; A2=total area of other openings in the air damper body through which air can bypass the air flow openings; A3=total area between an outer diameter (OD) of the air damper body and an inner diameter (ID) of the fire tube through which air can bypass the air flow openings, and FR=firing rate in % of maximum rate for a particular firetube-burner combination; DF=diameter factor, where D=fire tube inner diameter and the factor is a relationship of 0.12D̂2−6.29D+92.
 12. An air damper for a natural draft fire tube, comprising: an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings, with air flow through the air damper body being controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate, the air damper body having an air inlet face, an air outlet face, and an outer circumference; and a deformable circumferential seal around the outer circumference of the air damper body, the circumferential seal being adapted to engage an inner circumference of a fire tube solely by friction.
 13. The air damper of claim 12, wherein an ignitor passage extends through the air damper body, wherein an ignitor can be inserted and removed, a seal being provided to prevent airflow through the ignitor passage bypassing the air damper. 